Merck Checkpoint Inhibitor Shows Activity in Hard-to-Treat Form of Breast Cancer

NEW YORK (TheStreet) -- Five of 27 patients with an aggressive and hard-to-treat form of breast cancer responded to treatment with Merck's (MRK) checkpoint inhibitor Keytruda, according to interim results from an early-stage study announced Wednesday.

One of the five responding patients had a complete remission, which means there was no evidence of the cancer in her body. The other four patients had partial responses.

All the patients in the study had triple-negative breast cancer -- one of the most aggressive forms of the disease for which no targeted therapies have been successfully developed to date.

Merck said it will start a phase II study of Keytruda in triple-negative breast cancer in the first half of next year, supported by the clinical activity seen in the phase I study. Wednesday's Keytruda data were presented at the 2014 San Antonio Breast Cancer Symposium.

The class of immunotherapies known as checkpoint inhibitors work by blocking a protein known as PD-1 which cancer cells use to hide from a patient's immune system. Blocking PD-1 makes tumors visible to killer T cells.

Merck secured U.S. approval for Keytruda in advanced melanoma in September. Bristol-Myers Squibb's  (BMY) Opdivo is approved for melanoma in Japan and is under review for the same indication in the U.S. Both companies are rushing ahead to develop their respective drugs in other solid tumors like lung cancer, and now breast cancer. Both drugs are also showing promising clinical activity in blood cancers, highlighted at presentations this past weekend.

The Merck study enrolled 32 patients with triple-negative breast cancer that were determined to express the PD-1 protein. All the patients had advanced disease that was no longer responding to currently available treatments. The patients were then treated Keytruda.

Twenty-seven patients were evaluable for efficacy. Of those, five patients reported tumor shrinkage for an overall response rate of 18.5%. One patient had a complete response, the other four patients had partial responses to Keytruda. Overall 33% of patients achieved some level of tumor shrinkage.

At six months, 23% of patients were still alive with tumors that were not growing.

Two patients discontinued from the study due to adverse events tied to Keytruda. There was one patient who died due to Keytruda-related toxicity.

In a related announcement, Merck said it was teaming up with the International Breast Cancer Study Group and Breast International Group to conduct a study to investigate whether the combination of Keytruda and Roche's RHHBY Herceptin can be effective in patients with Her2-positive breast cancer  growing and no long responsive to Herceptin therapy.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.

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